1. Field of the Invention
This invention relates to treatment of wastewaters by the complete mix activated sludge process and particularly relates to utilization of this process in a single tank or basin for nitrification of ammonia in the wastewaters and for denitrification of the nitrites and/or nitrates formed therefrom.
2. Review of the Prior Art
Complete mix systems are designed so that if samples are taken simultaneously over the basin area, the measured properties are essentially uniform. As one of these properties, the dissolved-oxygen content (D.O.) is maintained as uniformly as possible at an average dissolved-oxygen content above one milligram per liter, so that aerobic conditions are continuously maintained in the treatment reactor or basin. These relatively constant and high-D.O. aerobic conditions are needed in order to maintain the normally required efficiencies of removing carbonaceous oxygen-demanding pollutants and nitrogenous oxygen-demanding pollutants from wastewaters.
In practice, the D.O. concentration is usually not uniform because higher D.O. concentrations are found closer to the aerators and to the liquid surface (particularly if surface aerators are used) and because lower D.O. concentrations are found near the sides and the bottom of the basin.
Complete mixing is commonly conducted in round, square, or rectangular tanks or basins into which incoming wastewater is fed at numerous places. The contents of the basins are thoroughly mixed to insure that the incoming wastewater is rapidly dispersed throughout the basins, in contrast to plug-flow systems. The volume of mixed liquor in a basin is so much greater than the volume of the incoming wastewater that the wastewater is overwhelmingly dominated by the basin contents. Also, there is a uniform concentration of mixed liquor-suspended solids (MLSS) to be found in complete mix aeration basins, as contrasted with the variable concentration noted in plug-flow and semi-plug flow tanks.
It should be understood that the mixed liquor in a complete mix activated sludge basin does not flow translationally, as in a smoothly flowing river or an oxidation ditch, wherein the translational flow is typically 1-3 feet per second. Instead, it moves onward very slowly, the hydraulic retention time within the basin typically being 6-60 hours, depending upon the strength of the incoming wastewater and the treatment requirements. However, it is not stagnant because the mixing devices move the liquor vertically, horizontally, and radially. A toroidal flow pattern around each mixing device is indeed a common occurrence so that each particle of mixed liquor is exposed repeatedly but randomly to contact with oxygen while the aerators are in operation.
Ammonia, derived from decomposition of proteins, is present in many wastewaters as a contaminant that must be removed because it is toxic to many forms of aquatic life at concentrations as low as one p.p.m. The mixture of microorganisms that exists in a barrier oxidation ditch is very well suited for, such removal by ammonia oxidation to nitrite with Nitrosomas (e.g., Nitrosomas europea), oxidation of nitrite to nitrate with Nitrobacter (e.g., Nitrobacter winnogradski and Nitrobacter agilis) and denitrification by reduction of the nitrite and/or nitrate to nitrogen gas with facultative heterotrophic microorganisms generally of the genera of Pseudomanas, Achromobacter, Bacillius, and Micrococcus. All of these microorganisms are ubiquitous in the environment. Both Nitrosomas and Nitrobacter require a dissolved oxygen level in excess of approximately 0.5 mg/l and preferably greater than 1.0 mg/l.
When operated with a constant high D.O. above one milligram per liter, the complete mix activated sludge process will provide high efficiency ammonia removal by biological nitrification. This continuous high D.O. process, however, does not have the ability to remove nitrites and nitrates that are produced in the nitrification process.
The cyclical complete mix activated sludge process that is disclosed in U.S. Pat. No. 4,917,805 of John H. Reid does provide a means whereby a complete mix activated sludge process can be operated to provide ammonia removal by nitrification and both nitrite and nitrate removal by biological denitrification. Biological denitrification is achieved in this cyclical complete mix activated sludge process by cycling the D.O. in the complete mix activated sludge basin in what was believed to be a sine wave pattern, similar, to the dissolved oxygen profile that would be experienced by bacteria circulating in a typical total barrier oxidation ditch activated sludge reactor.
Subsequent experimentation, however, demonstrated that the curve for the oxygen uptake rate in a total barrier oxidation ditch is steep and asymptotically decreases, and the curve for its oxygen consumption rate also shows asymptotic decrease. Moreover, it proved to be very difficult to control the air compressors in a complete mix system (especially in large basins having big air compressors) within close time sequences. For example, when an air compressor would be started by the plant operator., the D.O. level would automatically rise until the desired aerobic high D.O. level was reached at which time the air compressor intake valve would be throttled to maintain this desired aerobic level, but the amount of time required for the D.O. to rise and then drop again was too great because the centrifugal compressors slowed down rapidly but gradually arid, after being stopped for the anoxic cycle, started up again gradually with no surge, using a reduced voltage start followed by gradually increasing voltage. In other words, too much time was consumed in starting and stopping big compressors. On the other hand, if two CMAS system basins, supplied by the same centrifugal compressors, should be available, the process described in U.S. Pat. No. 4,917,805 should be practical.
A barrier oxidation ditch of such nitrification/denitrification capability operates on approximately a 6-18 minute cycle and contains microorganisms having a long sludge age or mean cell residence time (MCRT) involving much endogenous respiration in which cells die and lyse, releasing their nutrients which are consumed by other cells so that they become increasingly mineralized.
A barrier oxidation ditch must be operated entirely differently in summertime and in wintertime. When operating a barrier oxidation ditch during wintertime, denitrification becomes difficult when oxygen uptake becomes increasingly rapid as the mixed liquor becomes colder, the D.O. level tends to become increasingly greater, and the anoxic portion of the ditch tends to become increasingly shorter. In summertime, it can become quite difficult to attain a high enough D.O. level for the aerobic portion of the ditch while the biomass uptake and activity are greatly increased. In other words, not only weather, but also slug loads of food and activity of the biomass can affect the lengths of the aerobic and anoxic portions of the ditch. These factors are also of importance in other systems, such as CMAS basins.
Most attempts to accomplish cyclical oxic-anoxic GMAS basin operation and resulting nitrification-denitrification are believed to have utilized fill-and-draw sequence batch reactors (SBR). However, such sequence batch reactors, which do provide the closest approach to complete control of high and low D.O.'s that is believed to be achievable in a CMAS system, are operated mainly for settling of the sludge. The air must be shut off for awhile to permit such settling to occur, whereby the clarified liquor can be removed from the top portion of the tank and the settled sludge can be taken from its bottom, a portion of this sludge being removed from the complete mix basin as waste sludge. The practical drawback to SBR operation is the length of time required for settling, causing only 2-4 cycles/day (6 under exceptional circumstances) to be available.
Because the autotrophic microorganisms such as Nitrosomas and Nitrobacter grow much more slowly (for example, on the order of five to ten times more slowly) than the facultative heterotrophic microorganisms, an acclimation period of up to one to three months may be necessary, although maintaining a pH and temperature just below the maximum and a D.O. level just above the minimum can minimize this period.
As an example of processes adapted to cope with such differences in bacterial growth rates, the nitrification process disclosed in U.S. Pat. No. 4,705,633 increases the efficiency of nitrification by increasing the population of nitrifying bacteria beyond that which would naturally occur in a nitrifying activated sludge system by using a return sludge reaeration zone which is enriched with anhydrous ammonia or an aqueous solution thereof.
U.S. Pat. No. 3,342,727 describes a process for operating a CMAS basin while separating the mixing and aeration functions, holding mixing as an independent variable and aeration as a dependent variable. An agitator is mounted within the center of the basin, and air is supplied to a sparge ring beneath the agitator by an air blower. Attached to the side of the basin is a dissolved oxygen analyzer connected to a dissolved oxygen sensor beneath the surface of the mixed liquor. When changes in the biological food supply occur, a set point control unit effects operation of two control relays which suitably change the speed of the air blower so as to maintain the D.O. level between 1.5 and 2.5 parts per million. No provision is made, however, for process changes to compensate for changes in the weather.
The process of U.S. Pat. No. 4,537,682 controls the microorganism population by controlling the sludge wastage rate, hydraulic residence time, dissolved oxygen level, sludge mixing rate, biological oxygen demand, pH, and temperature for high-strength ammonia-containing wastewaters, possibly containing other contaminants such as phenolic, cyanide, and thiocyanide compounds, in order to nitrify and denitrify in a single reactor. Although it is true that this process is directed to the unusually difficult problem of treating high-strength industrial wastewaters, its seven areas of testing and control impose an onerous burden on a plant operator. Simpler methods of control, particularly for sanitary wastewaters and for wastewaters from food processing plants, are accordingly needed.
Other denitrifying methods also seek to remove phosphorus in addition to nitrogen, as exemplified by U.S. Pat. No. 4,655,925 which discloses a method of removing nitrogen and phosphorus from wastewater by using a mixed liquor comprising the wastewater and activated sludge within a single basin in which are aerating and mixing devices which cannot independently maintain a fixed mixing rate while selectively varying the oxygen transfer rate. However, in attempting to accomplish biological phosphorus removal by a process that includes an anaerobic cycle in which the microorganisms release phosphates to the wastewater and an aerobic cycle in which there is luxury uptake of phosphorus, it is extremely important not to have excessive sludge age in order to prevent cell breakdown and phosphorus release back to the wastewater liquid, resulting in increased effluent phosphorus concentrations. If a long sludge age is allowed to occur in the reactor, then more endogenous respiration will also occur, resulting in cell breakdown and release of stored phosphorus into he wastewater liquid so that there is increased effluent phosphorus concentration.
The cyclical activated sludge process disclosed in U.S. Pat. No. 4,655,925 and in other patents, such as U.S. Pat. No. 4,999,111, must therefore be limited to short Mean Cell Residence Times (MCRT's) in order to accomplish high efficiency biological phosphorus removal. If such removal is attempted through very accurate control of MCRT but at relatively low levels of MCRT in order to avoid difficulties with endogenous respiration and cell breakdown, it can become very difficult to attain a high enough MCRT to accomplish biological denitrification during the winter season. A cyclical CMAS process for a single basin that relates to nitrification and denitrifications only is accordingly needed.
Japanese Patent No. 53-9255l describes a method for waste treatment in a mixed liquor containing 15,000 mg/l or more of microorganisms by alternate reactions in two stages to adjust the D.O. to 0.4 mg/l or less and to 2 mg/l or more while BOD removal, nitrification, and denitrification occur simultaneously in the same reaction vessel. The control methods include cessation of air supply or of inlet wastewater. However, it discloses no method for making adjustments to compensate for changes in: (1) temperature of the wastewater and mixed liquor as the weather, changes, (2) quantity of the food supply, or (3) biomass uptake.